I'm a musician as well, and I think I can relate a lot to what he is going through. Unfortunately, performing can be a very nerve-wracking, scary thing, but it's just one of those things that you can only really get a comfort level with and used to with experience. Confidence and a positive frame of thinking are so powerful, as with most things in life... people tend to reflect and respond to what you present them. I've screwed up hardcore in performances before (in my head) but just kept going and played it off and just made a point to present this air of confidence..... no one but me even noticed otherwise. It's almost like your mindset sets up a sort of self-fulfilling prophecy. It's still hard to not be self- conscious at times, even the best musicians, no matter how talented, go through the same thing, and you can sort of end up shooting yourself in the foot.
I would just encourage him, remind him of this,, give him a lot of support and reassurance and whatever else to up his confidence.... encourage him to perform in front of people,,, even in baby steps with a smaller number of people that he is comfortable with amd build up from there..... I am very sure that over time he will eventually build up the confidence to be able to show off and express his god-given talents without the anxiety interfering with his gift
